1. Field of the Invention
The present invention relates to a radiographic apparatus including a radiation imaging unit and, more particularly, to a radiographic apparatus and a radiographic method for controlling a radiographic process based on a state of the radiation imaging unit.
2. Description of the Related Art
Radiation imaging sensors have been suggested in recent years, which are capable of converting radiographic images into digital data in real time. For example, it has become possible to manufacture solid-state photodetectors in which solid-state photodetector elements are arranged in a matrix. This leads to the introduction of radiation imaging sensors each having the solid-state photodetectors and phosphors for converting radiation into visible light, the solid-state photodetectors and the phosphors being alternately layered. Each of the solid-state photodetector elements has a transparent conductive film and a conductive film with an amorphous semiconductor sandwiched therebetween on a substrate made of quartz. With such a radiation imaging sensor, image data can be acquired in the following manner. That is, the radiation imaging sensor that is irradiated with radiation transmitted through an object converts the radiation into visible light with the phosphors. The visible light is detected as electrical signals by photoelectric converters in the solid-state photodetector elements. The electrical signals are read out from the solid-state photodetector elements by a predetermined reading method and undergo analog-to-digital conversion for providing image signals. Such a solid-state photodetector is disclosed in Japanese Patent Laid-Open No. 8-116044. Furthermore, radiation imaging sensors have also been suggested which directly acquire radiation by using solid-state photodetectors without using phosphors.
The radiation imaging sensors described above detect the radiation intensity as the amount of electric charge. Hence, it is necessary to drive the radiation imaging sensor in a stable state in order to accurately accumulate radiation signals. For this purpose, a driving operation, such as discharge of dark current or discharge of stored charge, is performed.
However, applying radiation having a high intensity causes stored signal charge in the solid-state photodetector element to be saturated. After the signal charge is saturated, it is impossible to fully discharge the electric charge by the driving operation in an ordinal radiographic process, thus superimposing noise on the image data. The noise produced based on the storage state of the electric charge or the discharge state of the electric charge in the radiation imaging sensor is called a residual image. Such a problem is also pointed out in Japanese Patent Laid-Open No. 2000-023968. A countermeasure is suggested for eliminating the residual image caused by residual charge with optical reset in this publication. However, with such a method, the radiographic imaging sensor must have a reset device, thus undesirably requiring the space and cost of the reset device. In addition, adding a driving operation for fully discharging the residual charge between radiographic processes in preparation for the application of high-intensity radiation disadvantageously lengthens the radiographic cycle and thus lowers the turnover of radiography in a hospital or the like.
Applying high-intensity radiation sometimes superimposes the noise caused by the residual charge on the image data. Hitherto, various countermeasures against the superimposition of the noise have been expected.